Fusion cages, as well as related interbody devices, are frequently utilized in spinal surgery between vertebrae of a patient. In particular, one or a pair of interbody devices are placed between the vertebrae to provide support and promote fusion between vertebrae where such is necessary due to disease, injury, general deterioration or a congenital problem. Frequently, the anterior sides or fronts of the vertebrae also require additional spacing in comparison to posterior sides. Therefore, it is often desirable to use an anteriorly expandable interbody device so that the vertebrae are spread or spaced more on the anterior sides thereof than on the posterior sides thereof. It is seldom if ever desirable to space the posterior sides of the vertebrae more than the anterior sides thereof. Interbody devices which provide for anterior greater expansion are generally referred to as interbody expansion devices or expansion cages. These cages are specifically expandable on the anterior end thereof, so that the fronts of the adjacent vertebrae between which the cages are located are more greatly spaced than the rears.
It is noted that interbody devices, such as fusion cages, may be inserted into the intervertebral space anteriorly or posteriorly. The present application is directed to the types of devices that are inserted anteriorly or from the front of the patient and are generally referred to as anterior interbody devices or anterior fusion cages.
Interbody devices typically must be very strong along the entire length of the top and bottom walls of the device that engage the vertebrae. In particular, in the turning and twisting of everyday life, substantial forces can be exerted against the interbody devices and, in particular, against one end or the other thereof. This is even more the case when the devices are subjected to unusual forces during an accident or the like. Some types of interbody devices in the prior art have provided for anterior expansion, but have not well supported the anterior end of the upper and lower walls. Such prior art devices have sometimes provided some kind of a wedge or rod that is urged axially along a ramp of the device in such a way as to expand the anterior portion subsequent to the cage being placed between the vertebrae. Because the wedge moves rearwardly or posteriorly toward the back wall of the device in order to spread the walls, a lever arm of unsupported wall is formed between the front edge of each wall and the position where the wedge engages the walls. Substantial forces can act on this lever arm. The interbody devices poorly supported along the unsupported wall can then fail due to forces exerted along the lever arm in the region.
Secondly, it is desirable that the interbody devices do not expand sideways while they are expanding vertically. This need arises because it is often necessary to put interbody devices in close proximity to one another in side by side relationship. Sideways expansion may prevent desirable positioning of the interbody devices and may also interfere with positioning of bone chips for grafting between the interbody devices.
When installing interbody devices, such as fusion cages, or even after an initial expansion a surgeon may also find that it is necessary to have more anterior expansion than was initially anticipated. Consequently, it is often desirable to have a variable expansion cage be able to increase the expansion in increments that allow the surgeon to determine if expansion is sufficient with each advance in expansion and thereafter go to a greater expansion, if necessary.
Finally, interbody devices, such as fusion cages, are quite expensive to produce in general. This is because the body of the devices must be made to very high tolerances in order to provide reproducible results and to provide the strength necessary to support the spine of the person. Because the surgeon is not always certain exactly which expansion size interbody device will be required until the surgery site is opened, it has been necessary for the surgeon to have on hand many different sizes of fusion cages and, in some cases, different types of fusion cages so as to insure that the necessary item will be present when the surgery is performed.
Consequently, it is also desirable to be able to provide an interbody device that is modular in nature in that it may be used as efficiently with no expansion as with expansion and the same device can be used with different degrees of expansion. Many of the expandable types of prior art fusion cages cannot be used as non-expandable type cages.
Further, it is desirable that a single fusion cage body be usable for virtually any degree of expansion desired or foreseeable by simply providing a set of comparatively much less expensive expansion members, any of which may be used in conjunction with the body to provide for various and different degrees of expansion.
An interbody device or fusion cage for use between a pair of adjacent vertebrae in order to provide support and/or promote growth between the vertebrae that have been destabilized due to injury, illness or the like. The interbody device includes a body which may be of different types, including bodies rectangular in shape, in which case the device is slid or driven between the vertebrae, or generally cylindrical in shape and threaded, in which case the device is screwed between the vertebrae. The body has an upper wall and a lower wall that are connected by a rear or posterior wall. The body is hinged about the rear wall by utilization of a material of construction that is flexible or resilient such as stainless steel or titanium, so that the upper and lower walls are able to pivot relative to each other at the rear, when a spreading force is applied to the walls, so that anterior ends of the walls rotate from an initial non-expanded configuration anteriorly to an expanded configuration anteriorly. Unless a spreading force is applied, the upper and lower walls are urged to remain generally parallel to each other.
The upper and lower walls also have legs on either side of the anterior end thereof that face toward similarly positioned legs on the other wall and which abut against each other when the device is in the unexpanded configuration thereof. An aperture is formed between the legs and preferably extends through the body in such a manner so as to form an interior chamber suitable for receiving bone chips or other growth promoting media. The body also preferably has upper and lower windows which communicate with the chamber and open onto the surface of the bones, when in use, so as to promote growth of the bone through the interbody device. The rear wall of the body includes a threaded bore.
An expansion member, preferably having a shape similar to a large headed bolt, is utilized to apply spreading force to the upper and lower walls so as to expand the body anterior end. The point or region of engagement between the expansion member and cage walls preferably remains the same throughout the expansion process by use of a wedge surface on the expansion member and is located near the anterior end of the cage. The expansion member includes an elongate shaft threaded on one end so as to be operably and threadably received in the rear wall bore and having a head at an opposite end. The shank includes a stop, preferably a section of the shank of enlarged diameter compared to and adjacent to the threaded end, which insures that the expansion member is properly positioned during use.
The expansion member head includes a first wedge surface that slopes radially outward and frontwardly relative to the remainder of the shank near an anterior end of the shank. The wedge surface preferably has the shape of a truncated cone. The wedge surface is sized, and shaped in position so as to engage a wedge mating edge or surface located on each of the lower and upper walls. In particular, the wedge surface first engages the mating surfaces on the walls at the posterior end of the wedge surface and then the mating surface slides along the wedge surface as the expansion member is screwed into the rear wall bore. In this manner the anterior end of the body is forced to spread or space vertically until a forward or anterior end of the edge of the wedge surface is reached. The wedge surface is adjacent to a generally cylindrical shaped support surface and transfers the support of the upper and lower walls to the cylindrical shaped surface as the expansion member is further rotated clockwise and advanced into the rear wall bore. The cylindrical support surface is preferably coaxial with an axis of rotation of the expansion member such that continued advancement of the expansion member to the stop causes the support surface to pass between the anterior ends of the upper and lower walls and to thereafter provide support to the upper and lower walls and keep the walls in a selected spaced relationship relative to each other.
In certain embodiments of the invention wherein the interbody device is rectangular in shape, a pair of side by side cages may be integrally joined or joined together by fasteners in order to form a single operative unit which preferably requires a pair of expansion members, but which may also use a single expansion member.
Furthermore, the devices, when used in side by side pairings, are frequently joined by a connector so as to further assure the stability of the devices. Also preferably, the connector extends between a relatively hard bony region associated with the anterior ends of the vertebrae so as to provide extra support in that region and resist subsidence of the vertebrae relative to the interbody devices.
Finally, in use each interbody device is normally provided with a set of expansion members wherein each member of the set provides a different degree of spacing of the anterior end of the interbody device. In this manner, a surgeon can utilize the interbody device without an expansion member or can alternatively select from a number of different sized expansion members to provide appropriate expansion of the anterior end of the interbody device. Normally, the surgeon would start with no or minimal expansion and then increase incrementally toward greater expansions by removal of less expanding devices and replacement by greater expanding devices until the surgeon is satisfied with the expansion provided.
Therefore, the objects of the present invention are: to provide an interbody device or fusion cage for use between a pair of vertebrae that is expandable; to provide such an interbody device that has a body and at least one expansion member wherein the expansion member or body has a wedge surface that engages an anterior portion of upper and lower walls of the body so as to spread the body from near the anterior end thereof; to provide such a device wherein a single body may be utilized either without an expansion member or in conjunction with any of an alternative group of modular expansion members each producing a different degree of expansion contained in a kit of expansion members; to provide such a device wherein an expansion member providing one degree of expansion can be removed and an expansion member providing a greater degree of expansion can be subsequently inserted; to provide such a device wherein the expansion members include an anterior or frontward cylindrical surface upon which anterior portions of the upper and lower body walls rest subsequent to full insertion of the expansion member; to provide such a device wherein the device expands vertically and not horizontally or side to side; to provide such a device wherein a pair of units may be joined into a single device and expanded by one or more expansion members; to provide such a device including a central cavity and windows to allow for packing with bone chips or other growth media so as to promote fusion between adjacent vertebrae exposed to the windows; to provide such a device wherein a single body may be utilized with a number of different expansion members, such that bodies of multiple different sized are not required to be maintained in stock during a surgical operation installing such devices; to provide such a device that does not cantilever the walls over a wedge that is medially located with respect to the body, but rather positions the wedge surface and the support surface at all times during expansion near the anterior end of the body so as to continuously provide support to anterior ends of the walls; to provide such a device that does not expand horizontally as the device expands vertically; and to provide such a device which is relatively inexpensive to produce, extremely easy to use and especially well adapted for the intended usage thereof.
Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention.
The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.